Flying craft capable of flying at hypersonic speeds, which have, for example, been investigated in the so-called "Saenger-Project" pose a substantial problem when it comes to the selection of a suitable engine. Compared to standard military aircraft and commercial aircraft, one must pay particular attention to the substantially larger range of flying speeds and the higher range of flying altitudes in combination with the corresponding variations in the environmental conditions such as pressure, temperature, and so forth. Further, such craft are expected to have a substantial flying range so that also the engine efficiency takes on a critical role.
It has been recognized heretofore that a single type of engine is not suitable for meeting all the above mentioned requirements in a satisfactory manner. As a result, combination systems with two or more different types of engines combined cannot be avoided. It is sensible to use air breathing engines for relatively low to moderate flying altitudes where a sufficient atmospheric air pressure is available because such air breathing engines can use the air oxygen as an oxidator. Turbo air jet engines have a special appeal for flying speeds in the range from subsonic to supersonic. One special type of a turbo air jet engine which is relatively independent from the environmental conditions is the so-called "air-turbo-rocket". This type of engine drives the compressor by means of a turbine which is independent of the atmospheric air. Propulsion gases from a rocket combustion chamber are admitted to the turbine for making it independent of the atmospheric air. Compared to conventional gas turbine engines the air turbo rocket is thus suitable for higher flying speeds as well as for higher flying altitudes. However, the air-turbo-rocket has a poor efficiency in the subsonic speed range.
So-called ram jets or scram jets are suitable for even higher flying speeds at altitudes where still sufficient atmospheric oxygen is available to sustain a subsonic or supersonic combustion.
For the highest flying speeds independent of atmospheric air, rocket engines are required which may be constructed as so-called mainstream rockets or bypass flow rockets. However, such rocket engines make sense only for high altitude flights, particularly in outer space because the oxidator for the rocket engine must be taken along as a ballast.
German Patent Publication (DE-PS) 3,617,915 discloses a conventional combination engine comprising a rocket engine section, a ram jet engine section, and a turbo air jet section forming a so-called "air-turbo-rocket".
The drive turbine of the compressor of the air turbo rocket serves also as the engine or drive for the fuel pumps of the rocket engine. Thus, it is necessary to provide a clutch or coupling between the turbine and the compressor for interrupting the power transmission between the turbine and the compressor during rocket operation. The turbo air jet engine section and the ram jet engine section are integrated into a common air jet engine and comprise a common flow channel for the inflowing external air. During ram jet operation the axial low pressure compressor is decoupled from its drive turbine and during this operation the compressor blades are tilted into a position in which they pose a low resistance, a so-called feathering position. The compressor thus must be equipped with adjustable compressor blades. Additionally, the compressor must be constructed with due regard to the high power transduction or throughput, with regard to the substantial size ratios and the high r.p.m.s and so forth, so that the compressor is a rather very critical structural component with regard to its material and structural strength. Additionally, due to the required adjustability of the compressor blades the structure becomes noticeably heavier compared to compressors with stationary non-adjustable blades. Due to the high power that must be transmitted between the turbine and the compressor, the respective clutch or coupling is also a critical structural component having a substantial weight. As a result, the compressor construction with adjustable blades has an adverse effect on the manufacturing costs and on the maintenance costs, not to mention the weight which reduces the payload. Besides, the complicated construction reduces the reliability.
German Patent Publication (DE-OS) 3,738,703 discloses a combination engine comprising a twin cycle gas turbine jet engine combined with a ram jet engine. In this known system the outer flow circuit of the gas turbine jet engine corresponds to the flow circuit of the ram jet engine. When the system operates as a turbo engine, the air in the outer flow circuit is accelerated by means of a fan having two rotor wheels without any guide blades or vanes and operating in opposition to each other. The drive of the two rotor wheels is accomplished by two turbines located in the inner flow circuit of the gas turbine jet engine. During ram jet operation the inner flow circuit is closed and the blades of the fans are brought into the feathering position.
Compared to the system of German Patent Publication (DE-PS) 3,617,915, the system disclosed in German Patent Publication (DE-OS) 3,738,703 does not require any clutch between the turbine and the fan. However, two fan rotor wheels are required and these wheels must be provided with adjustable blades which also leads to the same disadvantages as has been mentioned above. Further, a twin cycle gas turbine jet engine is substantially more complicated, heavier and more trouble-prone than a so-called air turbo rocket. Further, the system according to German Patent Publication (DE-OS) 3,738,703 is not operable independently of the atmospheric air. In other words, the turbo operation and the ram jet operation depend on the atmospheric air, so that such a system is not suitable for very high flying altitudes.